Systems and method for limiting maximum voltage in solar photovoltaic power generation systems

ABSTRACT

Apparatuses and methods are disclosed for regulating or limiting the voltage output from solar modules connected in series such that the voltage on a string bus connecting those solar modules does not exceed regulatory or safety limitations. This can be accomplished via a controller, local management units (for downconverting solar module voltage output), or a combination of the two.

RELATED APPLICATIONS

The present application claims the benefit of Provisional U.S.Application Ser. No. 61/273,210, filed Jul. 30, 2009 and entitled“SYSTEM AND METHOD FOR LIMITING MAXIMUM VOLTAGE IN SOLAR PHOTOVOLTAICPOWER GENERATION SYSTEMS,” which is incorporated by reference.

FIELD OF THE TECHNOLOGY

At least some embodiments of the disclosure relate to photovoltaicsystems in general, and more particularly but not limited to, improvingphotovoltaic energy generation.

BACKGROUND

The number of solar modules connected in series in a solar array, andthus the total output power of the system, is limited by safetyregulations. For instance, in the United States, the voltage on any partof the power line connecting solar modules into a solar array should notexceed 600V. In Europe this limit is 1000V. Conventional solar modulesmay typically generate a current and voltage that depend primarily onthe intensity and wavelengths of sunlight (e.g., twilight sees decreasedphoton intensity, mornings see a larger number of high-energy bluephotons, and cold temperatures increase solar cell efficiency and thusvoltage output, to name a few). As a result, solar modules may typicallygenerate a varying amount of power. To prevent solar arrays fromexceeding regulatory or other safety limits, solar modules may bedesigned to operate at voltages, and may be combined in limited numbers,that are well below regulator or safety limits. This buffer allows solararrays to stay below regulatory or safety limits even when solar modulesgenerate higher-than-average voltages. Thus, conventional solar arrayson average may typically generate less power (current and voltage) thanregulatory or safety limits, and may be limited in the number of solarmodules that can be connected in series with an inverter and/or combinerbox (or string combiner).

SUMMARY OF THE DESCRIPTION

Systems and methods in accordance with the present invention aredescribed herein. Some embodiments are summarized in this section.

In one of many embodiments of the present invention, apparatuses includean energy production system having a string bus and first and secondsolar modules. The first solar module may be connected to the string busand may generate a first voltage. The second solar module may beconnected to the string bus and may generate a second voltage. Theenergy production system may also include a controller. The controllermay be configured to limit the first voltage provided to the string bus,or to limit the second voltage provided to the string bus.

In another embodiment, apparatuses include an energy production systemhaving a string bus, a solar module, and a controller. The solar modulemay be connected to the string bus and may generate a voltage. Thecontroller may be in communication with the solar module and the stringbus. The controller may be configured to control the voltage provided tothe string bus. This control may be based on a predicted future voltage.

In another embodiment, a method includes monitoring a first voltage anda second voltage. The first voltage may be monitored across a firststring bus section that connects a first solar module to a second solarmodule. The second voltage may be monitored across a second string bussection that connects the second solar module to a voltage output. Themethod may further include limiting the voltage output by limiting atleast one of a voltage of the first solar module and a voltage of thesecond solar module.

Other embodiments and features of the present invention will be apparentfrom the accompanying drawings and from the detailed description whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are illustrated by way of example and not limitation inthe FIGS. of the accompanying drawings in which like references indicatesimilar elements.

FIG. 1 a illustrates an embodiment of an energy production system.

FIG. 1 b illustrates another embodiment of an energy storage system

FIG. 2 illustrates an energy production system where the voltageprovided to the string bus from each solar module is controlled by alocal management unit.

FIG. 3 illustrates an energy storage system where the controllercomprises local management units.

FIG. 4 illustrates an energy storage system where local management unitsreside on the solar modules.

FIG. 5 illustrates a solar module having a plurality of solar cellscontrolled by one or more local management units.

FIG. 6 illustrates a portion of an embodiment of an energy storage unitcomprising a solar module, a local management unit, and a portion of astring bus.

FIG. 7 illustrates a portion of another embodiment of an energy storageunit comprising a solar module, a local management unit, and a portionof a string bus.

FIG. 8 illustrates a method for carrying out the functions of thesystems herein disclosed.

FIG. 9 illustrates another method for carrying out the functions of thesystems herein disclosed.

FIG. 10 illustrates another method for carrying out the functions of thesystems herein disclosed.

DETAILED DESCRIPTION

The following description and drawings are illustrative and are not tobe construed as limiting. Numerous specific details are described toprovide a thorough understanding. However, in certain instances, wellknown or conventional details are not described in order to avoidobscuring the description. References to one system, method, device,apparatus, component, etc., or to one embodiment, are not necessarilyreferences to the same embodiment. Furthermore, such references mean atleast one.

The number of solar modules (or panels) in a solar array may be limitedby the maximum regulatory or safety voltage allowable on a string busconnecting those modules in series. It is desirable to operate a solararray at an open loop voltage (output of the entire array) as close toregulatory and safety limits as possible. Conventional systems havesolar modules whose voltage output are typically difficult to control,and thus operate well below regulatory and safety limits in order toprovide safety margins. The number of solar modules in an array is alsolimited for the same reason. The present disclosure overcomes theselimitations by limiting (controlling or regulating) the voltage thateach solar module provides to an array's output voltage. As such, solarmodules can operate at higher voltages, with smaller safety buffers toregulatory and safety limits, and more solar modules can be connected inseries in an array, since in times of excess energy generation moduleoutput can be reduced.

One embodiment of the present disclosure provides methods and systems tomonitor the voltages along a string bus connecting a plurality ofseries-connected solar modules, and limit a portion of the voltage thateach solar module provides to the string bus. In an embodiment, thesystems and method herein disclosed determine what portion of a solarmodule's voltage to provide to the string bus based on a trend in thesolar module's voltage. In this manner, a history of the solar module'sgenerated voltage can be used to determine whether a predicted futurevoltage output from a solar module to the string bus will cause thevoltage across any portion of the string bus to exceed a thresholdvoltage (e.g., regulatory limit associated with a particular solarmodule). In an embodiment, the systems and methods automatically limitthe voltage output from solar modules when the voltage across anyportion of the string bus exceeds another voltage threshold (e.g., avoltage just below of the regulatory or safety limit).

In an embodiment, the above-described operations can be controlled bylocal management units (LMU's) at each solar module (e.g., between eachsolar module and the string bus). The LMU's can be controlled via acentral controller (or a controller). In another embodiment, LMU's ateach solar module can control each solar module voltage output, whileone of the LMU's acts as a central controller for all of the LMU's.

The above-described controls also allow control of the open loop voltage(VOC) by, for example, switching off some or all solar moduleconnections to the string bus. For example, by using the LMU's of thesystem, one or more solar modules can be disconnected from the stringbus if any portion of the string bus voltage approaches a regulatory orsafety limit. As such, solar modules can operate at higher averagevoltages and more solar modules can be installed in a solar array (asolar array is an embodiment of an “energy production system”).

FIG. 1 a illustrates an embodiment of an energy production system. Theenergy production system 100 includes one or more solar modules 101(1),101(2), 101(3), . . . , 101(n) (n is any positive integer), a string bus110, and a controller 150. For the purposes of this disclosure, a “solarmodule” means a device comprising one or more solar cells connected inseries or parallel. Solar cells are configured to absorb and convertphotons into electricity. While solar cells can be designed to operatewith visible and near-visible wavelength photons, solar cells can alsoabsorb and convert to electricity photons having other wavelengths. Inan embodiment, each solar module 101(1), 101(2), 101(3), . . . , 101(n)absorbs photons and generates electricity. This will be referred to asvoltage generation. That voltage can be provided to the string bus 110.However, that voltage can also be limited (regulated, controlled,decreased, downconverted) and only a portion of the voltage generated bya solar module may be provided to the string bus 110. For instance, thefirst solar module 101(1) may generate a first voltage V₁ (e.g., 30V),but the voltage V₁′ provided to the string bus 110 may be only a portionof V₁ (e.g., 20V).

For the purposes of this disclosure, a “string bus” means a conductivemedium (e.g., wire, cable, lead, to name a few) configured to carryenergy from the solar modules 101(1), 101(2), 101(3), . . . , 101(n) toa voltage output 120. The one or more solar modules 101(1), 101(2),101(3), . . . , 101(n) can be connected to the string bus 110 (alsoreferred to as a string or serial bus string). In an embodiment, thesolar modules 101(1), 101(2), 101(3), . . . , 101(n) can be connectedserially to the string bus 110. In an embodiment, the string bus 110 cancarry signals. For instance, a signal can be modulated on the current orvoltage traveling through the string bus 110 to or from the solarmodules 101(1), 101(2), 101(3), . . . , 101(n). Such signals canrepresent data such as voltages and currents at different points in thesystem or instructions/commands for limiting solar module voltage outputto name a few.

Each solar module 101(1), 101(2), 101(3), . . . , 101(n) can provide avoltage to the string bus 110. For example, the first solar module 102can provide a first voltage V₁′ to the string bus 110; the second solarmodule 104 can provide a second voltage V₂′ to the string bus 110; thethird solar module 106 can provide a third voltage V₃′ to the string bus110, etc. The voltage on a segment (or portion) of the string bus 110between any two solar modules 101(1), 101(2), 101(3), . . . , 101(n) isequal to the sum of the voltage contributions from each solar modulethat came before that segment. For instance the voltage on the stringbus 110 between the second solar module 101(2) and the third solarmodule 101(3) is V₂. This voltage V₂ is equal to the sum of the firstvoltage V₁′ and the second voltage V₂′. In other words, V₂=V₁′+V₂′. Thiscan also be written as V₂=V₁+V₂′. An output voltage V_(O) is thus thesum of the voltage contributions from all solar modules in the system100 (V_(O)=V₁′+V₂′+V₃′+ . . . +V_(n)′).

The voltage that each solar module 101(1), 101(2), 101(3), . . . ,101(n) provides to the string bus 110 can be controlled (limited orregulated) by the controller 150. As a result, the controller 150 may beable to prevent a voltage on the string bus from exceeding a predefinedlimit. In an embodiment, the predefined limit can be a regulatory orsafety limitation. In the illustrated embodiment, the controller 150 isconnected to each solar module 101(1), 101(2), 101(3), . . . , 101(n).In an embodiment, the controller 150 can monitor currents and voltageson the string bus 110 and currents and voltages provided to the stringbus 110 by the solar modules 101(1), 101(2), 101(3), . . . , 101(n). Inan embodiment (not illustrated), the controller 150 can wirelesslycommunicate with the solar modules 101(1), 101(2), 101(3), . . . ,101(n). For the purposes of this disclosure, a “controller” means adevice that is an intelligent master to other subordinate devices. Forinstance, a solar module may be generating 30V, but the controller 150may instruct the solar module to provide only 20V to the string bus 110.In this manner, the controller 150 can ensure that the voltage on anypart of the string bus 110 does not exceed a threshold (or voltagethreshold). In an embodiment, this threshold is related to a regulatoryvoltage limit (e.g., 600V in the United States and 1000V in Europe). Inan embodiment, the threshold is slightly lower than the regulatory limitthus providing a margin of error relative to the regulatory limit orbuffer. In an embodiment, this threshold is related to a safety voltagelimit.

In an embodiment, the voltage provided to the string bus 110 by a solarmodule 101(1), 101(2), 101(3), . . . , 101(n) may be limited by one ormore switchable connections. The switchable connections may be coupledbetween each solar module 101(1), 101(2), 101(3), . . . , 101(n) and thestring bus 110. A switchable connection may comprise a switch, a gate, atransistor, or any other device configured to limit the current orvoltage passing between the solar modules 101(1), 101(2), 101(3), . . ., 101(n) and the string bus 110. The word “limit” should not beconstrued to mean a complete on or off state. In some embodiments, thismay be true (e.g., a mechanical switch). However, in some embodiments,switches only decrease the current or voltage.

The voltage output 120 can be connected to any number of devices orother energy transporting mediums (e.g., power lines, other buses, toname a few). Optionally, the voltage output 120 can be connected to aninverter 140, or to a string combiner 130 and an inverter 140. Thestring combiner 130 may also be known as a fuse box or chock box. Theinverter 140 can provide power to an electric grid, to a battery, or tosome other energy-using device or system. In one embodiment, thecontroller 150 is part of the inverter 140 or the string combiner 130.

FIG. 1 b illustrates another embodiment of an energy storage system. Theillustrated embodiment of energy storage system includes a controller150 connected directly to the string bus 110. The controller 150 canmonitor voltages via the string bus 110. The controller 150 can alsocommunicate instructions and/or data regarding the voltages to the solarmodules 101(1), 101(2), 101(3), . . . , 101(n) via the string bus 110,via wireless connection, or via both. The controller 150 can alsocommunicate instructions and/or data regarding the voltages to the LMU's(not illustrated) via the string bus 110.

Although the controller 150 connects to a bottom line of the string bus110, it should be understood that such a configuration is non-limiting.For instance, the controller 150 can be connected to the string bus 110in series or in parallel. The controller 150 can also communicate withthe inverter 140 or the string combiner 130 via the string bus 110,wireless connections, or a combination of both.

While the controller 150 illustrated in FIGS. 1 a and 1 b is connectedto the solar modules 101(1), 101(2), 101(3), . . . , 101(n), it will beseen in the following discussion of FIGS. 2-4 that other embodiments ofthe controller 150 are also possible.

FIG. 2 illustrates an energy production system where the voltageprovided to the string bus from each solar module is controlled by aLMU. Similarly to FIG. 1 a, the energy storage system 200 includes oneor more solar modules 201(1), 201(2), 201(3), . . . , 201(n) (n is anypositive integer), a string bus 210, and a controller 250. The energystorage system 200 also includes LMU's 202(1), 202(2), 202(3), . . . ,202(n) coupled between the solar modules 201(1), 201(2), 201(3), . . . ,201(n) and the string bus 210. For the purposes of this disclosure, a“LMU” means a device configured to limit (or regulate or manage orcontrol) the voltage that a solar module provides to a string bus. A LMUmay be variously referred to as a solar module controller (or converter)or link module unit. In an embodiment, the LMU's 202(1), 202(2), 202(3),. . . , 202(n) can limit a portion of the voltages provided to thestring bus 210 from each solar module 201(1), 201(2), 201(3), . . . ,201(n). For instance, the last solar module 201(n) may generate avoltage V_(n)′=50V. When this 50V is added to the string bus, the outputvoltage V_(O) may exceed the threshold value (a regulatory or safetylimit). Thus, instead of allowing all 50V to be provided to the stringbus 250, the LMU 202(n) may limit a portion of the voltage provided tothe string bus 210 to V_(n)″. As a result, the output voltage V_(O) willremain below the threshold voltage. Thus, one sees that each LMU 202(1),202(2), 202(3), . . . , 202(n) acts as a voltage converter capable ofdown converting each solar module voltage output.

In an embodiment, the LMU's 202(1), 202(2), 202(3), . . . , 202(n) limitthe voltage that the solar modules 201(1), 201(2), 201(3), . . . ,201(n) provide to the string bus 210 via a switchable connection. Forthe purposes of this disclosure, a “switchable connection” is aconnection between two conductors that can be opened and closed. Inother words, a switchable connection is one in which current can beselectively allowed to pass or not. Switchable connections oftencomprise a switch such as a gate or transistor. In an embodiment, aswitchable connection has two states, on and off. In an embodiment, theon state passes 100% of the current and voltage. In an embodiment, theon state passes slightly less than 100% of the current and/or voltage.In an embodiment, the off state passes 0% of the current and voltage. Inan embodiment, the off state passes slightly greater than 0% of thecurrent and/or voltage.

It should be understood that down conversion is not required. If thereis no need to limit the output voltage from a solar module 201(1),201(2), 201(3), . . . , 201(n), then the associated LMU 202(1), 202(2),202(3), . . . , 202(n) need not limit the voltage output. In anembodiment, the LMU's 202(1), 202(2), 202(3), . . . , 202(n) may beturned off when voltage limiting is not required.

In the illustrated embodiment, the controller 250 is connected to theLMU's 202(1), 202(2), 202(3), . . . , 202(n). In an embodiment, thecontroller 250 controls the LMU's 202(1), 202(2), 202(3), . . . ,202(n). In an embodiment (not illustrated), the controller 250communicates to the LMU's 02(1), 202(2), 202(3), . . . , 202(n) viawired connection, wirelessly (not illustrated), or both.

In an embodiment (not illustrated), the controller 250 can monitor thevoltages V₁′, V₂′, V₃′, . . . , V_(n)′ provided to the LMU's 202(1),202(2), 202(3), . . . , 202(n) from each solar module 201(1), 201(2),201(3), . . . , 201(n). In an embodiment, the solar modules 201(1),201(2), 201(3), . . . , 201(n) can monitor the voltages V₁′, V₂′, V₃′, .. . , V_(n)′ provided to the LMU's 202(1), 202(2), 202(3), . . . ,202(n) from each solar module 201(1), 201(2), 201(3), . . . , 201(n).The solar modules 201(1), 201(2), 201(3), . . . , 201(n) can communicatedata regarding the voltages V₁′, V₂′, V₃′, . . . , V_(n)′ to thecontroller 250 via a direct wired or wireless connection or via theLMU's 202(1), 202(2), 202(3), . . . , 202(n). Alternatively, the LMU's202(1), 202(2), 202(3), . . . , 202(n) can monitor the voltages V₁′,V₂′, V₃′, . . . , V_(n)′ and provide data regarding the voltages V₁′,V₂′, V₃′, . . . , V_(n)′ to the controller 250. The controller 250 orthe LMU's 202(1), 202(2), 202(3), . . . , 202(n) can also monitor thevoltage outputs V₁″, V₂″, V₃″, . . . , V_(n)″ from the LMU's 202(1),202(2), 202(3), . . . , 202(n) (the voltage provided to the string bus210).

In an embodiment, another device, such as a current/voltage monitoringdevice, can monitor currents and voltages. For instance, thecurrent/voltage monitoring device can monitor currents and voltages onthe string bus, or currents and voltages generated by the solar modules.The current/voltage monitoring device can then communicate datarepresenting the monitored currents and voltages to LMU's or thecontroller (depending on the embodiment).

In an embodiment, the controller 250 is configured to predict a voltagecontribution to the string bus 210 for each solar module 201(1), 201(2),201(3), . . . , 201(n). The controller 250 may be further configured todetermine if the predicted voltage contribution for each solar module201(1), 201(2), 201(3), . . . , 201(n) exceeds a predefined voltagelimit associated with a solar module. In one embodiment, the predefinedvoltage limit may be a value unique to each solar module 201(1), 201(2),201(3), . . . , 201(n). The controller 250 may be further configured toidentify each solar module 201(1), 201(2), 201(3), . . . , 201(n) andthe associated local management unit 202(1), 202(2), 202(3), . . . ,202(n) having a predicted voltage contribution exceeding the predefinedvoltage limit. The controller 250 may be further configured to instructeach identified local management unit to limit the voltage contribution.

In an embodiment, the controller 250 is configured to predict a voltageacross a portion of the string bus 210 spanning between two solarmodules or LMU's. The controller 250 may further be configured todetermine if the predicted voltage exceeds a voltage limit threshold.

In an embodiment, a controlling local management unit is configured topredict a voltage contribution to the string bus 210 for each solarmodule 201(1), 201(2), 201(3), . . . , 201(n). The controlling localmanagement unit may be further configured to determine if the predictedvoltage contribution for each solar module 201(1), 201(2), 201(3), . . ., 201(n) exceeds a predefined voltage limit. The controlling localmanagement unit may be further configured to identify each solar module201(1), 201(2), 201(3), . . . , 201(n) and the associated LMU 202(1),202(2), 202(3), . . . , 202(n) having a predicted voltage contributionexceeding the predefined voltage limit. The controlling LMU may befurther configured to instruct each identified local management unit tolimit the voltage contribution.

It should be understood that various methods and functions can becarried out by various components in the systems illustrated in FIGS.1-7. For instance, either the controller, the solar modules, or thelocal management units can monitor currents and/or voltage provided tothe string bus from the solar modules or can monitor currents and/orvoltages on the string bus. The controller, the solar modules, the localmanagement unit, or a controlling local management unit can analyze dataregarding currents and/or voltages. From this analysis, the controller,the solar modules, the local management units, or the controlling localmanagement unit can determine how to control voltages provided to thestring bus so as to prevent voltages on the string bus from exceedingregulatory or safety limits.

FIG. 3 illustrates an energy storage system where the controllercomprises LMU's. Similarly to FIG. 2, the energy storage system 300includes one or more solar modules 301(1), 301(2), 301(3), . . . ,301(n) (n is any positive integer), a string bus 310, and LMU's 302(1),302(2), 302(3), . . . , 302(n). However, the controller is not separatefrom the LMU's 302(1), 302(2), 302(3), . . . , 302(n), but rathercomprises them. In the illustrated embodiment, the LMU's 302(1), 302(2),302(3), . . . , 302(n) carry out functions that a controller might carryout including, but not limited to, monitoring voltages, determiningwhich voltages provided to the string bus 310 should be limited,predicting future voltages, comparing voltages to voltage thresholds,and limiting the voltages provided to the string bus 310 by each solarmodule 301(1), 301(2), 301(3), . . . , 301(n). Again, either the solarmodules 301(1), 301(2), 301(3), . . . , 301(n) or the LMU's 302(1),302(2), 302(3), . . . , 302(n) can monitor voltages. In determiningwhich voltages from solar modules 301(1), 301(2), 301(3), . . . , 301(n)should be limited, the LMU's 302(1), 302(2), 302(3), . . . , 302(n) maycommunicate with each other either via wired connection, wirelessly (notillustrated), or both.

In an embodiment, one of the LMU's 302(1), 302(2), 302(3), . . . ,302(n) can act as the controller. As such, the controlling LMU 302(1),302(2), 302(3), . . . , 302(n) can determine the voltages that the otherLMU's 302(1), 302(2), 302(3), . . . , 302(n) should provide to thestring bus 310. The controlling LMU 302(1), 302(2), 302(3), . . . ,302(n) can also perform all analyses in determining which LMU's 302(1),302(2), 302(3), . . . , 302(n) should limit voltages provided to thestring bus 310. The controlling LMU can be selected using any suitableprotocol. In one embodiment, the first LMU that announces its intent totake control of other LMU's can become the controlling LMU.

Alternatively, the LMU's 302(1), 302(2), 302(3), . . . , 302(n) can dothis monitoring and analysis in accord or individually. For instance,one LMU can monitor the voltage output of the solar module that the LMUis connected to. One LMU can also monitor the voltages on the string bus310. This same LMU can then determine how much voltage provided by thesolar module should be provided to the string bus 310, and limit thevoltage accordingly. Such operations can take place independent of theother LMU's 302(1), 302(2), 302(3), . . . , 302(n).

FIG. 4 illustrates an energy storage system where LMU's reside on thesolar modules. Similarly to FIG. 3, the energy storage system 400includes one or more solar modules 401(1), 401(2), 401(3), . . . ,401(n) (n is any positive integer), a string bus 410, and LMU's 402(1),402(2), 402(3), . . . , 402(n). The controller is again embodied by theset of LMU's 402(1), 402(2), 402(3), . . . , 402(n). However, in thisembodiment, the LMU's 402(1), 402(2), 402(3), . . . , 402(n) areincorporated into the solar modules 401(1), 401(2), 401(3), . . . ,401(n). From the solar modules, 401(1), 401(2), 401(3), . . . , 401(n)the LMU's 402(1), 402(2), 402(3), . . . , 402(n) are able to limit thevoltages V₁′, V₂′, V₃′, V_(n)′ provided to the string bus 410. Eitherthe solar modules 401(1), 401(2), 401(3), . . . , 401(n) or the LMU's402(1), 402(2), 402(3), . . . , 402(n) can monitor voltages. The LMU's402(1), 402(2), 402(3), . . . , 402(n) can work together orindependently. The LMU's 402(1), 402(2), 402(3), . . . , 402(n) cancommunicate with each other via wired connections, via wirelessconnection, or via both.

In each of the embodiments illustrated in FIGS. 1-4 it should beunderstood that communications between components can be performed atleast via the following three methods alone or in combination: wiredconnection, wireless connection, or the string bus. Multiple signals canbe communicated via a single connection (e.g., multiplexing).

FIG. 5 illustrates a solar module having a plurality of solar cellscontrolled by one or more LMU's. In one embodiment, the solar module 500has one or more strings of solar cells 506. For example, in theillustrated embodiment, there are at least two strings of solar cells506. In FIG. 5, a LMU 504 can control the voltage output of a group ofcells 502. In an embodiment, a LMU 504 can control the voltage output ofindividual cells 502. A string of solar cells 506 may be connected inseries, in parallel, or in a mesh configuration. The LMU 504 can controlthe voltage output of the string 506 or two or more LMU's 504 can beconnected in series to form a string. The string can be connected tooutput connections for the solar module 500.

FIGS. 6-7, illustrate LMU's according to some embodiments. In FIGS. 6-7,LMU's 602 may be configured to switch on and off the solar module 601periodically to limit the voltage provided to the string bus 610 fromeach solar module 601. One example of a LMU 602 is any of the variousLMU's (solar module controllers) offered by Tigo Energy, Inc. of LosGatos, Calif.

FIG. 6 illustrates a portion of an embodiment of an energy storage unitcomprising a solar module, a LMU, and a portion of a string bus. In FIG.6, a LMU 602 is local to the solar module 601 and can be used toperiodically couple the solar module 601 to the string bus 610 via theswitch Q1 606. By periodically switching switch Q1 606, the voltageprovided to the string bus 610 can be limited. The string bus 610 may ormay not be part of an overall mesh configuration of solar modules 601.

The switch Q1 606 can be switched at a particular duty cycle. For thepurposes of this disclosure, a “duty cycle” is the amount of time that aswitch is closed (i.e., passing current). For instance, a duty cycle of25% provides about a quarter of the solar module's 601 voltage to thestring bus 610 since, and assuming a one second period, the switch isclosed for 0.25 seconds and open for 0.75 seconds. As another example, a100% duty cycle provides about 100% of the solar module's 601 voltage tothe string bus 610 since the switch is continuously closed andconnecting a solar module to the string bus 610.

The LMU 602 may include a local controller 609 to control connectivityto the string bus 603. In an embodiment, the local controller 609controls connectivity to the string bus 603 via the switch Q1 606. Suchcontrol may be based on parameters such as duty cycle 604 a, phase 604b, and synchronization pulse 604 c. In one embodiment, the command tocontrol the operation of the switch Q1 606 is sent to the LMU 602 overthe photovoltaic (PV) string bus (power line) 610. Alternatively,separate network connections can be used to transmit the data and/orcommands to/from the LMU 602. Wireless communications are also possible.

The switch Q1 606 duty cycle can be adjusted by the LMU 602 based onmeasurements taken by the LMU 602. Alternatively, the duty cycle can beadjusted by the LMU 602 based on measurements taken by one or more otherLMU's or by a controller 150 (as in FIG. 1 a). The LMU 602 is an exampleof a switchable connection.

It should be understood that the details of the LMU 602 can beimplemented in the LMU's of FIGS. 1-3. For instance, in an embodimentcombining FIG. 2 and FIG. 6, the controller 250 monitors the duty cyclesof the LMU's 202(1), 202(2), 202(3), . . . , 202(n) and communicatesdata and/or signals representing the duty cycles to the LMU's 202(1),202(2), 202(3), . . . , 202(n). Alternatively, the LMU's 202(1), 202(2),202(3), . . . , 202(n) can communicate duty cycles to each other anddetermine their duty cycles based on those of the other LMU's.Alternatively, the LMU's 202(1), 202(2), 202(3), . . . , 202(n) cancommunicate duty cycles to a single LMU acting as the controller. Thatsingle LMU can then determine appropriate duty cycles for each LMU andcommunicate instructions for the other LMU's to operate at thedetermined duty cycles.

The LMU 602 may receive inputs 604 a, 604 b, 604 c, which areillustrated separately. However, the inputs 604 a, 604 b, 604 c are notnecessarily communicated to the LMU 602 via separate connections. In oneembodiment the inputs 604 a, 604 b, 604 c may be received in the LMU viaa single wired connection. In one embodiment, the inputs 604 a, 604 b,604 c are received in the LMU 602 via the string bus 610.

In one embodiment, the local controller 609 receives the parameters 604a, 604 b, 604 c from another LMU via the string bus 610 or a separatedata communication connection (e.g., a separate data bus or a wirelessconnection, to name a few). In an embodiment, the local controller 609receives the parameters 604 a, 604 b, 604 c from a controller such asthat depicted in FIG. 1 a. In some embodiments, the local controller 609may determine a parameter (e.g., 604 a and 604 b) based on the operatingparameters of the solar module 601 and/or measurements obtained by thelocal controller 609 without communicating with other LMU's or acontroller.

The LMU 602 may include a capacitor C1 605 to assist in filtering and/orensuring that the voltage provided to the string bus 610 is relativelyconstant. As illustrated in FIG. 6, the solar module 601 is connected inparallel to the capacitor C1 605.

The LMU 602 may include a diode D1 607 to prevent current from travelingbackwards in the string bus, for example in the case of a failure of thepanel connected to said LMU. As illustrated in FIG. 6, the diode D1 607is connected in series with the string bus 610. The switch Q1 606 of theLMU 602 can selectively connect or disconnect the solar module 601 andthe capacitor C1 605 from a parallel connection with the diode D1 607.In so doing, the switch Q1 606 connects or disconnects the solar module601 from the string bus 610. When the switch Q1 606 is on (closed), thesolar module 601 provides energy to the string bus 610 and is supportedby the capacitor C1 605 allowing a current larger than the current thatcould be provided solely by the solar panel. When the switch Q1 606 isoff (open), the solar module 601 does not provide energy to the stringbus 610 but rather the solar module 601 charges the capacitor C1 605 soit can discharge a portion of its energy during the next cycle. In otherwords, the capacitor C1 605 acts to smooth the voltage output whichwould otherwise have a square wave profile. In some cases, an activeswitch may be added in parallel to diode D1 607 to further enhance itsefficiency (not illustrated) Additional filters may be may be usedoutside the diode D1 607 to reduce noise in the string (e.g.,capacitors, resistors, inductors, or any combination of these, to name afew).

FIG. 7 illustrates a portion of another embodiment of an energy storageunit comprising a solar module, a LMU, and a portion of a string bus.The LMU 702 is connected between the solar module 701 and the string bus710 to control or limit the voltage provided to the string bus 710.Commands to the LMU 702 can be sent over the photovoltaic (PV) stringbus (power line) 710. The inputs 704 a, 704 b, 704 c to the localcontroller 709 were drawn separately, which does not necessarilyindicate that the inputs 704 a, 704 b, 704 c are provided via separateconnections and/or from outside the LMU 702. For example, in someembodiments, the local controller 709 may determine the parameters 704a, 704 b, 704 c based on measurements obtained at the LMU 702, with orwithout data from outside the LMU 702.

FIG. 7, like FIG. 6, includes a LMU 702 coupling the solar module 701 tothe string bus 710. The LMU 702 periodically connects and disconnectsthe solar module 701 to and from the string bus 710. The LMU 702 isparallel coupled to the solar module 701 and series connected to thestring bus 710. The LMU 702 can be serially connected to other LMU's.The LMU 702 has a switchable connection (e.g., switch Q1 706) configuredto connect and disconnect the solar module 701 to the string bus 710.The LMU 702 may receive, among others, three inputs or types of inputdata, including the following: (a) requested duty cycle 704 a, which canbe expressed as a percentage (e.g., from 0 to 100%) of time the solarmodule 701 is to be connected to the string bus 710 via the switch Q1706, (b) a phase shift 704 b in degrees (e.g., from 0 degree to 180degree) and (c) a timing or synchronization pulse 704 c. These inputs(e.g., 704 a, 704 b and 704 c) can be supplied as discrete signals, orcan be supplied as data on a network, or composite signals sent throughthe power lines (e.g., string bus 710) or wirelessly, and in yet othercases, as a combination of any of these input types.

In FIG. 7, the LMU 702 includes a capacitor C1 705 and the switch Q1706, as well as a diode D1 707. In FIG. 7, the diode D1 707 issupplemented with an additional switch Q2 708, which acts as asynchronous rectifier to increase efficiency. In one embodiment, theadditional switch Q2 708 is open (off) when the switch Q1 706 is closed(on) to connect the solar module 701 (and the capacitor C1 705) to thestring bus 710. When the switch Q1 706 is open (off), and the solarmodule 701 is charging the capacitor C1 705, the additional switch Q2708 can be closed (on) to divert the current on the string bus 710around the diode D1 707. In this fashion, losses from passing currentthrough the forward-biased diode D1 707 can be avoided.

In some cases, a filter (not shown), including a serial coil and aparallel capacitor, can be used. The filter may be placed at the LMU orplaced just before the fuse box or inverter, or be part of either one ofthose.

In FIG. 7, the controller 709 is used to process the input signals(e.g., 704 a, 704 b, 704 c) and drive the switches Q1 706 and Q2 708. Inthe illustrated embodiment, the controller 709 is a small single chipmicro controller (SCMC). For example, the controller 709 may beimplemented using Application-Specific Integrated Circuit (ASIC) orField-Programmable Gate Array (FPGA). The controller 709 can even beimplemented in discrete, functionally equivalent circuitry, or in othercases a combination of SCMC and discrete circuitry. It will be generallyreferred to as single chip micro controller (SCMC) herein, but anyimplementation may be used.

In one embodiment, the local controller 709 is coupled to the solarmodule 701 in parallel to obtain power for processing; and thecontroller 709 is coupled to the string bus 710 to obtain signalstransmitted from other LMU's coupled to the string bus 710, and tomonitor currents and voltages on the string bus 710.

In one embodiment, the switches in different LMU's can operate atdifferent phases to minimize voltage variance on the string bus. Forexample given two LMU's operating at a 50% duty cycle, the localcontroller of each LMU could be cycled 180 degrees (one half cycle) outof phase. As such, when one local controller opened the connection toits solar module, the other local controller closed the connection toits solar module. The result is a steadier supply of voltages to thestring bus than if the two solar modules were connected and disconnectedto the string bus at the same times.

In one embodiment, the local controller (SCMC) 709 is connected (notshown in FIG. 7) to the solar module 701 to obtain power for controllingthe switches Q1 706 and Q2 708. In one embodiment, the local controller(SCMC) 709 is further connected (not shown in FIG. 7) to the string bus710 to transmit and/or receive information from the string bus 710. Inone embodiment, the local controller (SCMC) 709 includes sensors (notshown in FIG. 7) to measure operating parameters of the solar module701, such as module voltage, module current, temperature, lightintensity, etc.

Returning now to the controller described throughout this disclosure,various means for determining when and how to limit the voltagesprovided to the string bus are possible. For instance, an energyproduction system may comprise a string bus, a solar module connected tothe string bus and generating a voltage, and a controller incommunication with the solar module and the string bus. The controllermay be configured to control what portion of the voltage is provided tothe string bus based on a predicted future voltage. For the purposes ofthis disclosure, a “predicted future voltage” is an estimated voltageexisting at a particular future time. For instance, the controller maymonitor the string bus and note that the string bus voltage is likely toexceed a regulatory limit within five minutes. The expected voltage infive minutes is called the predicted future voltage. As a result, thecontroller may limit the voltage contribution of one or more solarmodules to the string bus so that the string bus voltage remains belowthe regulatory limit, in some cases preferably in a balanced manner.

In an embodiment, the portion of a solar module's voltage provided tothe string bus may be roughly inversely related to the magnitude of thepredicted future voltage. In other words, the greater the predictedfuture voltage, the lower the voltage provided to the string bus.

The predicted future voltage can also be determined via a voltage trend.A voltage trend may include data or analysis of data regarding voltagesthat have been monitored. This historical data or trend data can be usedto estimate what the predicted future voltage will be.

In an embodiment, the voltage provided to the string bus may be limitedto a default voltage. In other words, when the string bus voltage orsome other monitored voltage becomes excessive or is predicted to becomeexcessive, the voltage provided to the string bus from one or more solarmodules can be limited to a predefined default value. For instance, whenthe string bus voltage approaches a regulatory limit, the voltage fromeach solar module provided to the string bus can be limited to 50% ofeach solar module's maximum output. Alternatively, when the string busvoltage or some other monitored voltage becomes excessive or ispredicted to become excessive, one or just a few solar modules can belimited to contributing 50% of their output voltage to the string bus.In an embodiment, a voltage threshold may be used to determine when oneor more solar modules should be limited to providing a default voltageto the string bus. For instance, given a voltage threshold of 590V onthe string bus, and due to a malfunction in the inverter, the string busvoltage exceeds 590V, the voltages that each solar module provides tothe string bus may be limited to a default value (e.g., 75% of output or50% of output or 25% of output, to name a few). In an embodiment,turning to the default voltage can be triggered by a hardware (e.g.,differential amplifier and a Zener diode threshold) or software safetymechanism. As an example, if the controller monitors a certain rate ofchange it may trigger commands to the LMU's to fall back to the defaultvoltage. These and similar methods allow the system, in extraordinarysituations (e.g., loss of load or partial loss of wiring) to maintainstring bus voltage below regulatory or safety limits.

In an embodiment, the controller can communicate with any other part ofthe energy production system via one or more of the LMU's. In anembodiment, the controller can monitor voltages on the string bus via aLMU. For instance, the controller can monitor voltage on the string busvia data gathered by each of the LMU's.

FIG. 8 illustrates a method for carrying out the functions of thesystems herein disclosed. The method 800 includes a monitor firstvoltage operation 802 in which a first voltage is monitored across afirst string bus section. The first string bus section includes aportion of the string bus connecting a first solar module to a secondsolar module. The first voltage will thus include the voltage providedto the string bus by the first solar module plus any voltage alreadycontributed to the string bus via upstream solar modules (downstreambeing the direction that current travels).

The method 800 also includes a monitor second voltage operation 804 inwhich a second voltage is monitored across a second string bus section.The second string bus section includes a portion of the string busconnecting a second solar module to a downstream solar module or anoutput voltage. As noted in FIG. 1 a, the output voltage can beconnected to any number of devices or systems including a stringcombiner, an inverter, a power grid, or power lines, to name a few. Thesecond voltage will thus include the voltage provided to the string busby the second solar module plus the voltage provided by the first solarmodule plus any voltage already contributed to the string bus viaupstream solar modules.

In an embodiment, the first and second monitor voltage operations 802,804 can operate simultaneously. In an embodiment, one of the twooperations 802, 804 can follow the other in time. In an embodiment, thefirst and second monitor voltage operations 802, 804 may operate overdifferent periods of time while a portion of those periods of time mayoverlap.

The method 800 also includes a limit first voltage or second voltageoperation 806. In an embodiment, the limit operation 806 may limit thefirst voltage by limiting the voltage output of the first solar module.With reference to FIGS. 8-10, voltage is measured on the string bus,while voltage output is the voltage provided to the string bus from asolar module. In an embodiment, the limit operation 806 may limit thesecond voltage by limiting the voltage output of the second solarmodule. In an embodiment, the limit operation 806 may limit the firstand second voltages, to the same or different values, by limiting thevoltage output of the first and second solar modules. Limiting voltageoutput can be performed via any of the systems and methods describedpreviously or with reference to FIGS. 1-7. Determining when and by howmuch voltage will be limited can also be performed via any of thesystems and methods previously described or with reference to FIGS. 1-7.

Although only two solar modules have been described with reference toFIG. 8, it should be understood that the disclosed method applies to anynumber of solar modules. It should also be understood that themonitoring operations 802, 804 and the limiting operation 806 do nothave to operate sequentially. In an embodiment, the limiting operation806 can be carried out while monitoring continues. In other words,monitoring can be a continuous process or discrete (currents andvoltages monitored at periodic intervals).

FIG. 9 illustrates another method for carrying out the functions of thesystems herein disclosed. The method 900 includes a monitor firstvoltage operation 902 in which a first voltage is monitored across afirst string bus section. The first string bus section includes aportion of the string bus connecting a first solar module to a secondsolar module. The first voltage will thus include the voltage providedto the string bus by the first solar module plus any voltage alreadycontributed to the string bus via upstream solar modules (downstreambeing the direction that current travels).

The method 900 also includes a monitor second voltage operation 904 inwhich a second voltage is monitored across a second string bus section.The second string bus section includes a portion of the string busconnecting a second solar module to a downstream solar module or anoutput voltage. As noted in FIG. 1 a, the output voltage can beconnected to any number of devices or systems including a stringcombiner, an inverter, a power grid, or power lines, to name a few. Thesecond voltage will thus include the voltage provided to the string busby the second solar module plus the voltage provided by the first solarmodule plus any voltage already contributed to the string bus viaupstream solar modules.

In an embodiment, the first and second monitor voltage operations 902,904 can operate simultaneously. In an embodiment, one of the twooperations 902, 904 can follow the other in time. In an embodiment, thefirst and second monitor voltage operations 902, 904 may operate overdifferent periods of time while a portion of those periods of time mayoverlap.

Once the first and second voltages have been monitored, the method 900compares the first and second voltages to a threshold voltage via adetermination operation 905. The determination operation 905 determineswhether the first or second voltages exceed the voltage threshold. Suchcomparison can be performed, for instance, as a backup safety measure.Normally, the system predicts future voltages, and can scale backvoltage output from solar modules in order to account for unusually highsolar module voltage generation. However, sometimes string bus voltagemay rise faster than the system can react to. In such an instance thedetermination operation 905 can lead to initiation of an automaticvoltage limitation—a backup safety measure. One or more of the solarmodules can automatically be instructed to limit output voltage to aspecified low level when the voltage threshold is exceeded. Thus, bysetting the voltage threshold at a level below a regulatory or safetylimit, the method 900 ensures that solar module voltage outputs will bequickly and significantly reduced if the string bus voltage gets tooclose to a safety or regulatory limit. In an embodiment, the thresholdvoltage may be different for each section of the string bus.

In an embodiment, when either the first or second voltages exceed thevoltage threshold, the voltage output of the first or second solarmodule (or both), whichever is generating excessive voltage, can belimited. In an embodiment, when either the first or second voltages (orboth) exceed the voltage threshold, the voltage output of the first andsecond solar modules can be limited. Such, an embodiment might be usedwhere greater safety is desired than in the embodiment where only selectsolar module output voltages are limited.

If the voltage threshold is exceeded, then one or both of the solarmodules can be regulated. As such, the method 900 includes a limit firstvoltage and second voltage operation 906. In an embodiment, the limitoperation 906 may limit the first voltage by limiting the voltage outputof the first solar module. In an embodiment, the limit operation 906 maylimit the second voltage by limiting the voltage output of the secondsolar module. In an embodiment, the limit operation 906 may limit thefirst and second voltages by limiting the voltage output of the firstand second solar modules. Limiting voltage output can be performed viaany of the methods and systems described in earlier paragraphs and withreference to FIGS. 1-7.

If the voltage threshold is not exceeded, then the method 900 can loopback to the monitor operations 902, 904. The monitor operations 902, 904can reinitiate after the determination operation 905, can automaticallyoperate at a periodic interval, or can continuously monitor voltages.Although only two solar modules have been described with reference toFIG. 9, it should be understood that the disclosed method applies to anynumber of solar modules.

FIG. 10 illustrates another method for carrying out the functions of thesystems herein disclosed. In the method 1000, the duty cycle of allLMU's (or their switchable connections, should they have them) can beadjusted in order to limit or regulate the string bus voltage. The dutycycles may be adjusted so that the voltage provided to the string bus donot cause the string bus voltage to exceed the maximum voltage allowed(e.g., regulatory or safety limits, to name two). For example, themaximum voltage may be limited by the string combiner 130, the inverter140, or any other load connected to the string bus 110, or limited byany regulations applicable to that system. In some embodiments, the dutycycles are adjusted to align the voltage of multiple strings.

To limit string bus voltage, the method 1000 includes a monitor firstvoltage operation 1002 and a monitor second voltage operation 1004. Themethod 1000 also includes a limit first or second voltages operation1006. The limit operation 1006 limits the voltage output from either thefirst or second solar modules depending on which one (or both) isgenerating a voltage that is or may cause the string bus voltage toexceed the maximum allowable voltage. This is done via adjusting theduty cycle of switchable connections coupling the solar modules to thestring bus. There is at least one switchable connection between eachsolar module and the string bus.

In one embodiment, the duty cycles are computed for the solar modulesthat are connected to a string bus via corresponding LMU's. The dutycycles can be calculated based on measured current and voltages of thesolar modules.

After an initial set of duty cycles is applied to the solar modules, theduty cycles can be further fine tuned and/or re-adjusted to changes incurrent and/or voltage. In one embodiment, target voltages are computedfor the solar modules, and the duty cycles are adjusted so that thevoltages provided to the string bus converge towards the targetvoltages. The methods to compute the duty cycles of the solar modulescan also be used to compute the duty cycles of the groups of solar cellswithin a solar module (recall FIG. 5).

Many variations may be applied to the systems and methods hereindisclosed without departing from the spirit of the invention. Forexample, additional components may be added, or components may bereplaced. For example, rather than using a capacitor as to smooth solarmodule voltage output, an inductor may be used, or a combination ofinductor(s) and capacitor(s). Also, the controller and/or LMU's cancomprise hardware, hardware and software, or software. Additionally, thecontroller can be further connected to a private network (e.g.,intranet) or the Internet. The controller could then communicate withother computers and servers. One application of such a connection wouldallow the controller to determine the local regulatory voltage limitsand modify voltage thresholds and voltage limiting algorithmsaccordingly to tailor the system to those local regulatory voltagelimits. Also, the balance between hardware and firmware in thecontrollers or LMU's can be changed without departing from the spirit ofthe invention. In case the controller or LMU's are not able tocommunicate with each other (e.g., during startup), the controller orLMU's may have a default voltage limit at which they operate untilcommunications can be established. The methods for determining the dutycycles for the solar modules can also be used to determine the dutycycles of groups of cells connected via LMU's in a string of solar cellswithin a solar module.

In one embodiment, the controller can be off the shelf and possiblymodified. In one embodiment, the controller can have analog circuitry.In one embodiment, the controller can be a microcontroller. In oneembodiment, the controller could be a combination of these features.

It is clear that many modifications and variations of this embodimentmay be made by one skilled in the art without departing from the spiritof the novel art of this disclosure. These modifications and variationsdo not depart from the broader spirit and scope of the invention, andthe examples cited here are to be regarded in an illustrative ratherthan a restrictive sense.

In the foregoing specification, the disclosure has been described withreference to specific exemplary embodiments. Various modifications maybe made thereto without departing from the broader spirit and scope ofthe disclosure. The specification and drawings are illustrative ratherthan restrictive.

1. An energy production system, comprising: a string bus; a first solarmodule connected to the string bus and generating a first voltage; asecond solar module connected to the string bus and generating a secondvoltage; a single controller configured to limit at least one of thefirst and second voltages provided to the string bus; a first localmanagement unit coupled between the first solar module and the stringbus, and in communication with the single controller; and a second localmanagement unit coupled between the second solar module and the stringbus, and in communication with the single controller.
 2. The energyproduction system of claim 1, wherein the controller controls the firstlocal management unit and the second local management unit.
 3. Theenergy production system of claim 1, wherein the controller comprises: afirst local management unit coupled between the first solar module andthe string bus; and a second local management unit coupled between thesecond solar module and the string bus.
 4. The energy production systemof claim 1, further comprising a switchable connection between: thefirst solar module and the string bus; and the second solar module andthe string bus.
 5. The energy production system of claim 4, wherein theswitchable connection includes at least one transistor.
 6. The energyproduction system of claim 5, wherein: the first voltage provided to thestring bus is controlled via a duty cycle parameter and a phaseparameter applied to the at least one transistor of the first localmanagement unit; and the second voltage provided to the string bus iscontrolled via a duty cycle of the at least one transistor of the secondlocal management unit.
 7. The energy production system of claim 1,wherein the controller prevents a voltage on the string bus fromexceeding a predefined limit.
 8. An energy production system,comprising: a string bus; a solar module connected to the string bus andgenerating a voltage; and a controller in communication with the solarmodule and the string bus, and configured to control the voltageprovided to the string bus based on a predicted future voltage aspredicted by the controller and a maximum regulatory safety voltage. 9.The energy production system of claim 8, wherein a connection betweenthe solar module and the string bus can be disconnected via a switch.10. The energy production system of claim 9, wherein the controlleradjusts the duty cycle of the switch.
 11. The energy production systemof claim 9, wherein the switch is a transistor.
 12. The energyproduction system of claim 8, wherein the predicted future voltage isbased on a voltage trend.
 13. The energy production system of claim 8,wherein the controller is further configured to limit the voltageprovided to the string bus to a default voltage.
 14. The energyproduction system of claim 13, wherein the default voltage is used whenthe voltage exceeds a voltage threshold.
 15. The energy productionsystem of claim 8, wherein the solar module is connected to the stringbus via a local management unit.
 16. A method comprising: monitoring afirst voltage across a first string bus section connecting a first solarmodule to a second solar module; monitoring a second voltage across asecond string bus section connecting the second solar module to avoltage output; and limiting the voltage output based on a maximumregulatory safety voltage by limiting at least one of a voltage of thefirst solar module and a voltage of the second solar module.
 17. Themethod of claim 16, wherein limiting comprises limiting the at least oneof the voltage of the first solar module and the voltage of the secondsolar module when the at least one of the voltage of the first solarmodule and the voltage of the second solar module exceeds a voltagethreshold.
 18. The method of claim 16, wherein the limiting comprises:controlling the first voltage by controlling a duty cycle of a firstswitchable connection coupled between the first solar module and thestring bus; and controlling the second voltage by controlling a dutycycle of a second switchable connection coupled between the second solarmodule and the string bus.
 19. The method of claim 16, wherein thelimiting is based on a predicted future voltage.